Silicone gel-coated wound dressing

ABSTRACT

A wound dressing product comprising: a substrate layer having an upper surface and a lower surface; a tacky silicone coating composition present on said upper surface and on said lower surface; and upper and lower release sheets covering said substrate and said tacky silicone coating composition on said upper and lower surfaces, respectively, and adhered to said surfaces by said tacky silicone coating composition, wherein said upper surface is less tacky than said lower surface whereby said upper release sheet can be removed from said upper surface more readily than said lower release sheet can be removed from said lower surface. The same silicone composition is used to form both the upper and the lower surface. Also provided are methods for making such dressings.

The present invention relates to wound dressings comprising a substratecoated on both surfaces with a tacky silicone gel, wherein the surfaceshave different tackiness, and to methods of making such dressings.

Dressing materials for application to the surface of wounds shoulddesirably be non-adherent to the moist wound surface, but sufficientlytacky to allow attachment of the dressing to intact skin around thewound and attachment to further dressing layers such as absorbentlayers. For this purpose a soft or tacky hydrophobic material issuitable for the material. The wound contacting material shoulddesirably be liquid-permeable to allow passage of wound fluid,especially for heavily exuding wounds such as bums. The material shouldalso be non-irritating, inexpensive, and stable to common sterilizationmethods such as ionizing radiation.

Traditional “tulle gras” dressings generally consist of a layer of gauzecoated with paraffin wax. Such dressings have a number of desirableproperties, and for this reason have been used extensively for manyyears. Among these advantages are their high degree of conformabilityand deformability, and the fact that their tackiness makes them veryeasy to apply. That is to say, a tulle gras dressing applied to a woundwill usually remain in place simply by adhesion of the paraffin wax tothe patient's skin (or to itself in the case of a dressing wrappedaround a finger, for example) while a securing bandage is applied. Tullegras dressings are also quite inexpensive. However, tulle gras dressingsdo have a number of disadvantages. Principal amongst these is that,although initially non-adherent, they often become “dry” (in the senseof losing their paraffin coating) and consequently adhere to the woundto which they are applied. This effect is due to the paraffin coatingbecoming mobile at body temperatures and migrating into the wound orbeing absorbed into the backing of the dressing or bandage. In somecases, removal of a tulle gras dressing which has become dry in this waycan cause considerable trauma. Indeed, it is quite common to have tosoak tulle gras dressings in order to remove them. If tulle grasdressings are changed more frequently, in an attempt to avoid thembecoming attached to the wound, this may delay wound healing and adds tonursing costs.

A further disadvantage of traditional tulle gras dressings is thatfibres from the gauze may become incorporated in the wound, as may theparaffin coating of the dressing. Some authorities see the migration ofparaffin into a wound as an undesirable effect and any paraffin found ina wound can be difficult to remove with normal aqueous wound cleansingagents. Moreover, the pores of the gauze may become occluded if theparaffin coating is too heavy or as a result of the mobility of theparaffin during use of the dressing. While occlusive dressings areappropriate in some circumstances, it is undesirable that the nursingstaff should have no control over whether the dressing used is in factocclusive.

Still further disadvantages of conventional tulle gras dressings arethat they are effectively opaque and of somewhat unsightly appearance,and the paraffin can run during storage, making them particularly messyto apply.

EP-A-0251810 describes wound dressing materials that overcome the abovedisadvantages by replacing the paraffin wax coating of conventionaltulle gras by a tacky or non-tacky, hydrophobic silicone coating on agauze or mesh substrate. In certain embodiments, the gauze may beprovided with a tacky silicone coating on one side and a non-tackysilicone coating having a different composition on the other side.Similar materials are described in WO-A-8705206.

EP-A-0342950 describes similar wound dressings having a non-adherentsilicone coating. The adherence of the silicone is reduced by additionof an amine-extended polyurethane.

U.S. Pat. No. 6,846,508 describes medical adhesive devices having tackysilicone layers on both surfaces of a substrate. The material of thetacky layers may be the same or different, and the tackiness of thelayers may be the same or different.

U.S. Pat. No. 5,891,076 describes scar dressings comprising a carriermaterial embedded in a layer of silicone gel which is tacky, and coversheets over the silicone gel layers.

JP-A-10095072 describes a double-sided silicone adhesive tape fornon-medical adhesive applications and having release sheets over thesilicone adhesive.

The present invention provides a wound dressing product comprising: asubstrate layer having an upper surface and a lower surface; a tackysilicone coating composition present on said upper surface and on saidlower surface; and upper and lower release sheets covering saidsubstrate and said tacky silicone coating composition on said upper andlower surfaces, respectively, and adhered to said surfaces by said tackysilicone coating composition, wherein said upper surface is less tackythan said lower surface whereby said upper release sheet can be removedfrom said upper surface more readily than said lower release sheet canbe removed from said lower surface.

Suitably, the substrate is porous, whereby the substrate is permeable tothe fluid. For example, the substrate may be a mesh or web or fabricsuitably formed from a woven, nonwoven or knitted textile or a moldedmesh.

In certain embodiments the substrate is a fabric such as a gauze, or amesh, having an array of apertures. The size and shape of the aperturesin the substrate are not critical, but the apertures should suitably besuch as to ensure that the material can be adequately coated withsilicone gel without them becoming occluded. The apertures generallyhave an aspect ratio of from 1:1 to 5:1, and preferably from 1:1 to 2:1.For example, the apertures may be approximately circular orapproximately square. The apertures suitably have an average diameter offrom 0.3 to 4 mm, and more suitably from 0.5 to 2 mm.

The substrate is suitably formed from any medically acceptable material,such as cellulose, polyolefins, polyesters, or polyamides. An especiallysuitable material is cellulose acetate gauze. Substrates having a weightof from 15 to 200 g/m² are generally found to be suitable for use in theproducts of the invention, and fabrics weighing from 50 to 150 g/m² aremost suitable. For example, certain embodiments employ a fabric of from80 to 120 g/m².

Suitably, the silicone-coated substrate product retains open aperturesto allow passage of wound fluid through the coated substrate. Forexample, an array of apertures may extend through said silicone coatingsand the substrate layer. The open area of the coated substrate in thefinal product may for example be from about 1% to about 70%, for examplefrom about 10% to about 50%.

The dressing materials of the invention are characterized by tacky upperand lower silicone coatings on the substrate, wherein the upper andlower silicone coatings have different tackiness. This provides theadvantage that both surfaces can be protected before use by cover sheetsadhered to the coating by the tackiness thereof, but one of the coversheets can be removed more easily than the other whereby application ofthe dressing is easier. Not only is it easier to selectively remove thefirst cover sheet if it is less strongly adhered than the second coversheet, but also the resulting exposed less-adherent surface is moresuitable for application to the wound surface. The second cover sheetcan then be removed to expose a more adherent surface for application ofsecondary dressing layers, such as absorbent layers.

The difference in tackiness between the upper and lower siliconecoatings is suitably selected to optimise the above properties. Forexample, the tackiness of the lower surface as measured by a loop tacktest (described below) is suitably from 5% to 150% greater than thetackiness of the upper surface, more suitably from 20% to 100% greater,for example about 30% to 70% greater. Suitably, the tackiness of thesurfaces as measured by the loop tack test is greater than about 0.3N.For example, they may be from about 0.4N to about 2N, more suitably fromabout 0.5N to about 1.5N. In embodiments, the tackiness of one surfaceis from about 0.4N to about 1N and the tackness of the other surface isfrom about 0.5N to about 1.5N.

Suitably, the above difference in tackiness can be achieved in materialswherein the silicone on said upper and lower surfaces is substantiallychemically homogeneous. That is to say, wherein the same siliconeprepolymer is coated onto the upper and lower surfaces, but differencesin coating or curing conditions are used to achieve different tackinessin the upper and lower surfaces. Thus, both surfaces have been formed bycuring the same fluid silicone prepolymer.

The term “chemically homogeneous” signifies that the chemical analysis(wt % silicon, carbon, oxygen, etc.) is the same on the upper and lowersurfaces, but the tackiness may be different due to different degrees ortype of cure between the surfaces and/or different coating thicknesseson the two surfaces. Thus, the silicone material forms a continuouschemically homogeneous phase having different tackiness on its opposingsurfaces.

The total coating weight of the tacky silicone (combined upper and lowerlayers) is suitably from about 50 g/m² to about 500 g/m², for examplefrom about 80 g/m² to about 200 g/m², typically from about 100 g/m² toabout 150 g/m². The silicone is suitably a soft skin adhesive siliconecomposition. Suitably chemistry is described below. The silicone issuitably hydrophobic.

The effectiveness of encapsulation by silicone means that the substratemay be printed or dyed with decorative or informative matter with littledanger of the ink or dye being released into the wound to which thedressing is applied. Visible indicia, such a colour or writing, may beprovided on one or both of the surfaces of the substrate and/or on oneor both cover sheets to indicate which sides of the product have themore/less tacky silicone coating and thus to indicate which cover sheetis removed first and which silicone surface is less tacky forapplication to the wound surface.

The cover sheets may comprise a film of polyethylene, polypropylene orfluorocarbons and papers coated with these materials. Suitably, thecover sheet is a release-coated paper sheet, such as a siliconerelease-coated paper sheet. Examples of silicone-coated release papersare POLYSLIK (Registered Trade Mark) supplied by H.P. Smith & Co.,offered in various formulations to control the degree of adhesion of thepaper to the adhesive surface.

In certain embodiments, one or both cover sheets may comprise two ormore parts, such as a first removable part having a first edge and asecond removable part that meets the first part along the first edge.Suitably, along each of said edges where the parts meet, one of theparts is folded back to provide a folded-back margin, and the other partoverlaps the said folded-back margin. This provides an easy-to-graspmargin on each part in the region of overlap to assist removal of thecover sheet by the care giver.

In other embodiments, one or both cover sheets may comprise three parts,for example as described in detail in EP-A-0117632.

The products of the invention may be made into wound dressings forapplication to the surface of a wound by removing the top and bottomcover sheets. Suitably, the products of the invention consistessentially of the substrate, the silicone coatings, and the coversheets. Suitably, the products of the invention are sterile and packagedin a microorganism-impermeable container.

In a further aspect, the invention provides a method of making a wounddressing material comprising:

-   -   providing a substrate layer having an upper surface and a lower        surface;    -   coating said upper and lower surfaces of said substrate layer        with a fluid silicone prepolymer composition; followed by    -   thermally partially curing said silicone prepolymer composition        to produce an intermediate material having a partially cured        silicone composition on said upper and lower surfaces; followed        by    -   storing said intermediate material at a temperature below 50° C.        for at least 2 days to allow equilibration of the silicone        coatings on the upper and lower surfaces; followed by    -   further curing said partially cured silicone composition by        exposing said intermediate material to ionizing radiation, to        produce a final material having tacky silicone coatings on said        upper and lower surfaces.

In a further aspect, the invention provides a method of making a wounddressing material comprising:

-   -   providing a substrate layer having an upper surface and a lower        surface;    -   coating said upper and lower surfaces of said substrate layer        with a fluid silicone prepolymer composition; followed by    -   thermally partially curing said silicone prepolymer composition        to produce an intermediate material having a partially cured        silicone composition on said upper and lower surfaces; followed        by    -   further curing said partially cured silicone composition by        exposing said intermediate material to ionizing radiation, to        produce a final material having tacky silicone coatings on said        upper and lower surfaces,    -   wherein either (a) said step of coating is applies unequal        weights of the silicone coating composition to said upper and        lower surfaces, and/or (b) said step of thermally partially        curing applies different amounts of heat to said upper and lower        surfaces, whereby said silicone coatings on said upper and lower        surfaces have different tackiness following said further curing        step.

The substrate layer is suitably as previously described for the productsof the invention. Suitably, the substrate layer is permeable to thefluid silicone prepolymer composition. The step of coating may beperformed in any conventional way, for example immersion, spraying, ordoctor blade. The step of coating suitably comprises passing the coatedsubstrate through nip rollers to ensure smooth coating and penetrationof the coating composition. Suitably, the step of coating is followed bya step of blowing gas (such as air) through the substrate to ensure thatthe apertures in the material are open after coating.

The fluid silicone coating composition is suitably substantially orcompletely solvent-free. Suitably, both surfaces are coated with thesame silicone coating composition. The methods of the present inventionallow products having different tackiness on the upper and lowersurfaces to be made with a single silicone coating composition on bothsurfaces.

Suitably, the silicone composition is a so-called soft skin adhesivesilicone elastomer. Such silicones can be made by an addition reaction(hydrosilylation) between (a) a vinyl functional polydimethyl siloxane,such as bis-dimethyl vinyl PDMS, and (b) a hydrogen functional siloxane,such as dimethyl, methylhydrogen siloxane copolymers, hydrogendimethylsiloxy terminated PDMS. The cure reaction is catalyzed by ahydrosilylation catalyst, such as a noble metal catalyst, suitably aplatinum catalyst. Suitably the silicone prepolymer composition furthercomprises a polymerization inhibitor that is evaporated from saidcomposition during said step of thermally partially curing, for example2-methyl-3-butyn-2-ol. The polymerization inhibitor is suitably presentin an amount of from about 0.001 wt. % to about 1 wt. %, for examplefrom about 0.01 wt. % to about 0.1 wt. % before curing.

Silicone skin adhesive compositions are suitably supplied as two-partsystems: Part A contains at least the vinyl prepolymer and the catalyst,while Part B contains the vinyl prepolymer and the SiH siloxane crosslinker. The components are mixed immediately before use, optionally withaddition of the polymerization inhibitor.

In embodiments, the silicone coating composition comprises or consistsessentially of the following components:

(A) a diorganopolysiloxane having at least 2 alkenyl groups in eachmolecule;(B) an organohydrogenpolysiloxane having at least 2 silicon-bondedhydrogen atoms in each molecule, in a quantity sufficient for the ratiobetween the number of moles of silicon-bonded hydrogen atoms in thiscomponent and the number of moles of alkenyl groups in component (A) tohave a value of from about 0.6:1 to about 20:1,(C) optionally a platinum group metal catalyst suitably in a quantityproviding 0.1 to 500 weight parts as platinum group metal per 1,000,000weight parts component (A); and(D) a volatile polymerization inhibitor, suitably selected from: alkynealcohols such as 20 2-methyl-3-butyn-2-ol, 3,5-dimethyl-1-hexyn-3-ol,and phenylbutynol; ene-yne compounds such as 3-methyl-3-penten-1-yne and3,5-dimethyl-3-hexen-1-yne; tetramethyltetrahexenyl-cyclotetrasiloxane;and benzotriazole.

The diorganopolysiloxane, component (A), used in the instant inventionis the base component of the total composition. Thisdiorganopolysiloxane must contain at least 2 alkenyl groups in eachmolecule in order for this composition to cure into a rubbery elasticsilicone rubber coating composition.

The diorganopolysiloxane (A) comprises essentially straight-chainorganopolysiloxane with the average unit formula R_(n)SiO_((4-n)/2),wherein R is selected from substituted and unsubstituted monovalenthydrocarbon groups and n has a value of 1.9 to 2.1. R may be exemplifiedby alkyl groups such as methyl, ethyl, propyl, and others; alkenylgroups such as vinyl, allyl, and others; aryl groups such as phenyl, andothers; and haloalkyl groups such as 3,3,3-trifluoropropyl and others.The diorganopolysiloxane (A) should have a viscosity at 25° C. of atleast 100 centipoise (1 d Pa·s). When such factors as the strength ofthe silicone rubber coating membrane, and blendability are taken intoaccount, the viscosity of diorganopolysiloxane (A) at 25° C. ispreferably from 1,000 centipoise (1 Pa·s) to 100,000 centipoise (100Pa·s). The diorganopolysiloxane (A) may be exemplified bydimethylvinylsiloxy-endblocked dimethylpolysiloxanes,dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxanecopolymers, and dimethylvinyl-siloxy-endblockeddimethylsiloxane-methylphenylsiloxane copolymers.

Component (B), an organopolysiloxane that contains at least 2silicon-bonded hydrogen atoms in each molecule, is a crosslinker for thecomposition of the instant invention. The organopolysiloxane (B) may beexemplified by trimethylsiloxy-endblocked methyl-hydrogenpolysiloxanes,trimethylsiloxy-endblocked dimethylsiloxanemethylhydrogen-siloxanecopolymers, dimethylphenylsiloxy-endblockedmethylphenylsiloxanemethyl-hydrogensiloxane copolymers, cyclicmethylhydrogenpolysiloxanes, and copolymers that contain thedimethylhydrogensiloxy unit and SiO4/2 unit. Theorganohydrogenpolysiloxane (B) should be added in a quantity that theratio between the number of moles of silicon-bonded hydrogen atoms inthis organohydrogenpolysiloxane and the number of moles of alkenylgroups in component (A) has a value of 0.6:1 to 20:1.

The platinum group metal catalyst, component (C), used in thecompositions is a curing catalyst. The platinum group metal catalyst (C)may be exemplified by platinum micropowder, platinum black,chloroplatinic acid, platinum tetrachloride, olefin complexes ofchloroplatinic acid, alcohol solutions of chloroplatinic acid, complexesbetween chloroplatinic acid and alkenylsiloxanes, rhodium compounds, andpalladium compounds. The platinum group metal catalyst (C) should beadded generally at 0.1 to 500 weight parts as platinum group metal per1,000,000 weight parts component (A), and is preferably used at 1 to 50weight parts as platinum group metal per 1,000,000 weight partscomponent (A). The reaction will not develop adequately at less than 0.1weight parts, while additions in excess of 500 weight parts areuneconomical.

The coated substrate is then subjected to thermal curing to partiallycure the silicone. The thermal coating is suitably performedcontinuously by passing the coated substrate through an oven. Suitablethermal curing conditions include exposure to a temperature of fromabout 80° C. to about 200° C., for example about 120° C. to about 180°C. for a time of from about 1 minute to about 10 minutes, for exampleabout 1.5 minutes to about 5 minutes. The elevated temperature resultsin evaporation of the polymerization inhibitor from the siliconecomposition and therefore in polymerization of the silicone. Theresulting material is chemically polymerized, but capable of furthercuring by ionizing radiation as explained further below.

The partially cured material has unequal tackiness of the siliconecoating on the upper and lower surfaces. This unequal tackiness can beproduced by (1) unequal coating weights of the silicone on the upper andlower surface, and/or (2) the step of blowing air through the coatedsubstrate to open the apertures, which results in a higher coatingweight of silicone on the surface that is downstream of the air flow,and/or (3) unequal amounts of heat supplied to the upper and lowersurfaces in the oven resulting in different degrees of cure on the twosurfaces.

The initial difference in tackiness of the upper and lower surfaces istypically rather greater than desired for the final product. However,the present inventors have found that the tackiness of the upper anlower surfaces gradually equilibrates if the partially cured material isstored at temperatures below about 50° C., for example ambient ornear-ambient temperatures such as 10-50° C., for a period of from about2 days to about 10 weeks. Therefore, suitably the partially curedmaterial is stored at such temperatures for a period of at least about 2days, suitably about 2 weeks to about 10 weeks, for example about 3weeks to about 8 weeks before the final cure with ionizing radiation.

Suitably, the method of the invention further comprises applying upperand lower release sheets over said silicone composition on the upper andlower surfaces. Suitably, the upper and lower release sheets are appliedintermediate said steps of thermally partially curing and storing,whereby, for example, the partially cured material having the coversheets can be rolled up for the storage step.

The partially cured material is then subjected to a final cure withionizing radiation. The ionizing radiation is suitably selected frome-beam radiation and gamma radiation. A variety of procedures for E-beamand gamma ray curing are well-known. The cure depends on the specificequipment used, and those skilled in the art can define a dosecalibration model for the specific equipment, geometry, and line speed,as well as other well understood process parameters.

Commercially available electron beam generating equipment is readilyavailable. For example, a Model CB-300 electron beam generatingapparatus (available from Energy Sciences, Inc. (Wilmington, Mass.).Generally, a support film (e.g., polyester terephthalate support film)runs through a chamber. Generally, the chamber is flushed with an inertgas, e.g., nitrogen while the samples are e-beam cured. Multiple passesthrough the e-beam sterilizer may be needed.

Commercially available gamma irradiation equipment includes equipmentoften used for gamma irradiation sterilization of products for medicalapplications. Cobalt 60 sources are appropriate. Total absorbed dosesare suitably from 20 to 60 kGy, more suitably from about 35 to 50 kGyand dose rates are suitably about 7 to 8 kGy/hour.

Suitably, the method further comprises the step of packaging theintermediate material in a microorganism-impermeable container prior tosaid step of further curing, and wherein said step of further curingalso sterilizes the material.

The methods of the invention may be used to make any products accordingto the invention. Any feature disclosed herein in relation to any one ormore aspects of the invention is suitable for use in any of the otheraspects defined herein.

Specific embodiments of the invention will now be described further, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a perspective view of a product according to the inventionpackaged in a microorganism-impermeable pouch;

FIG. 2 shows a perspective exploded view of a product according to theinvention;

FIG. 3 shows a top plan view partially cut away of the product of FIG.2;

FIG. 4 shows an enlarged partial cross-section through the product ofFIG. 2;

FIG. 5 shows a schematic view of an apparatus for making a productaccording to the method of the invention;

FIG. 6 shows a schematic diagram of the apparatus used for the loop tackmeasurement test; and

FIG. 7 shows a graph of measured loop tack adhesion for upper and lowersurfaces of materials made in accordance with the present invention,wherein the samples have been stored for different times prior to theirradiation curing step.

Referring to FIGS. 1 to 4, the product 1 according to the inventioncomprises a substrate 2 of cellulose acetate gauze of density 107 gramsper square meter nominal, having upper and lower surfaces 4,5 coatedwith a hydrophobic, tacky, crosslinked silicone gel. The siliconecomposition penetrates the gauze substrate to form a single, chemicallyhomogeneous silicone phase on the upper and lower surfaces. The coatedsubstrate 6 has an array of apertures extending through the substrateand the silicone to allow passage of wound fluid through the material.The tackiness of the coated upper surface 4 is approximately 50% greaterthan the tackiness of the coated lower surface 5, as determined by theloop tack test described below. The nominal total coating weight of thesilicone is 120-130 grams per square meter.

Identical release-coated cover sheets 7,8 are applied to the upper andlower silicone-coated surfaces 4,5. In use, the lower release sheet 8 isremoved first to expose the less tacky lower surface 5 of the dressingmaterial. It is relatively easy to selectively remove 25 the lowerrelease sheet 8 because of the lower adherency of this sheet to thematerial compared to the upper release sheet 7. The lower and/or upperrelease sheets may further comprise indicia to identify the releasesheet to be removed first. The lower surface 5 may then be applied to awound surface, followed by removal of the upper release sheet 7 andapplication of secondary dressing elements such as an absorbent layer.

Referring to FIG. 1, the product 1 is shown sterile and packaged in amicroorganism-impermeable envelope 3 having a transparent window 10.

Referring to FIG. 5, the process according to the invention starts froma continuous web of cellulose acetate gauze 12 that is passed through afluid silicone coating composition 14 and nip rollers 15 to coat andimpregnate the gauze with the silicone composition. The silicone coatingcomposition is prepared by mixing Components A and B of a soft siliconeskin adhesive silicone elastomer kit supplied by Dow Corning underproduct reference Q7-9177. The components are mixed in weight ratio50:50. Component A comprises a bis-dimethylvinyl terminatedpolydimethylsiloxane and a platinum catalyst. Component B comprises abis-hydride terminated polydimethylsiloxane. To the mixture is added2-methyl-3-butyn-2-ol inhibitor at a concentration of 0.02 wt. %.

The coated substrate then passes over a blower 17 to open the aperturesof the coated substrate that may have been occluded by the silicone.

The coated gauze is then passed through an oven 18 held at 150° C.Typical conditions are 5 passes at 4.2 m/min, total residence time 1.5minutes. This results in thermal partial cure of the silicone coating.The coated material is then allowed to cool, and release coated papercover sheets (not shown) are then continuously applied to the upper andlower surfaces at location 20 and the material is rolled up on roll 22for equilibration.

The rolls 22 of thermally cured and interleaved material are allowed toequilibrate at controlled temperature (20-25° C.) for 4-6 weeks. Thematerial is then cut and packaged as shown in FIG. 1, followed by gammairradiation with 35-50 kGy of Cobalt 60 radiation at 7-9 kGy/hr tosterilize the products and complete the cure. The irradiation curingresults in a further increase in both hardness and tackiness of thesilicone coating.

Procedure 1: Measurement of Surface Tackiness by the Loop Tack Test

The tackiness of the silicone coatings produced by the methods of theinvention was measured in a tensile tester, such as an Instron tester,using the set-up shown in FIG. 6.

Samples of the coated fully cured gauze having the release cover sheetsattached were cut to dimensions 5×9.5 cm. Margins of 1 cm were markedout along the long edges by drawing straight lines 1 cm from the longedges. The cover sheets were removed, and the sheet of coated gauze 30was looped around and the 1 cm margins 32,34 on opposed edges of onesurface (opposite the surface being measured) were applied firmly toopposite sides of a 2 mm thick metal spacer bar 38. Strips ofpolypropylene film 1 cm wide 40,42 were then applied to the oppositesurfaces of the coated gauze opposite the spacer bar 38 to prevent thecoated gauze from adhering to the jaws of the measurement device.

The assembly of polypropylene strips, coated gauze and spacer bar wasthen gripped in the jaws 44 of the Instron tester. The loop of coatedgauze 30 having the surface under test outermost was then lowered onto aclean polycarbonate surface 46 of dimensions 15.5 cm×3.8 cm so that theloop adheres to the surface, and raised to detach the loop from thesurface. Lowering and raising are performed at 300 mm/min, and theminimum distance between the jaws 44 and the polycarbonate surface 46 is15 mm. The measured tack (in Newtons) is the maximum force measuredwhile detaching the loop from the surface. Average of three measurementswas used.

FIG. 7 shows tack data obtained by the above for samples of the coatedgauze obtained in accordance with the invention. The coating weight was118 grams per square meter. Measurements were made on samples sterilizedafter storage at 25° C. and 4% relative humidity for 0, 2, 4, 6, and 8weeks. The upper and lower surfaces initially have very different tackas evidenced by the two groups of measurements at around 0.4N and 1.1N,but the tack converges on storage after about 4 weeks such that thefinal tack of the upper surface is about 1.0N and the final tack of thelower surface is about 0.7N.

The above examples have been described by way of illustration only. Manyother embodiments falling within the scope of the accompanying claimswill be apparent to the skilled reader.

1. A wound dressing product comprising: a substrate layer having anupper surface and a lower surface; a tacky silicone coating compositionpresent on said upper surface and on said lower surface; and upper andlower release sheets covering said substrate and said tacky siliconecoating composition on said upper and lower surfaces, respectively, andadhered to said surfaces by said tacky silicone coating composition,wherein said upper surface is less tacky than said lower surface wherebysaid upper release sheet can be removed from said upper surface morereadily than said lower release sheet can be removed from said lowersurface.
 2. A wound dressing material according to claim 1, wherein anarray of apertures extends through said silicone coatings and saidsubstrate layer.
 3. A wound dressing material according to claim 1,wherein said substrate layer comprises or consists essentially of awoven, nonwoven or knitted mesh.
 4. A wound dressing material accordingto claim 1, wherein the tackiness of said lower surface as measured by aloop tack test is at from 20% to 100% greater than the tackiness of saidupper surface.
 5. A wound dressing material according to claim 1,wherein said silicone on said upper and lower surfaces is substantiallychemically homogeneous.
 6. A wound dressing material according to claim1, wherein said silicone on said upper and lower surfaces has beenformed by curing of the same silicone prepolymer composition.
 7. A wounddressing material according to claim 1 which is sterile and packaged ina microorganism-impermeable container.
 8. A method of making a wounddressing material comprising: providing a substrate layer having anupper surface and a lower surface; coating said upper and lower surfacesof said substrate layer with a fluid silicone prepolymer composition;followed by thermally partially curing said silicone prepolymercomposition to produce an intermediate material having a partially curedsilicone composition on said upper and lower surfaces; followed byfurther curing said partially cured silicone composition by exposingsaid intermediate material to ionizing radiation, to produce a finalmaterial having tacky silicone coatings on said upper and lowersurfaces, wherein either (a) said step of coating is applies unequalweights of the silicone coating composition to said upper and lowersurfaces, and/or (b) said step of thermally partially curing appliesdifferent amounts of heat to said upper and lower surfaces, whereby saidsilicone coatings on said upper and lower surfaces have differenttackiness following said further curing step.
 9. A method according toclaim 8, further comprising storing said intermediate material at atemperature below 50° C. for at least 2 days to allow equilibration ofthe silicone coatings on the upper and lower surfaces.
 10. A methodaccording to claim 9, wherein said step of storing is carried out for atime of from 2 weeks to 10 weeks.
 11. A method according to claim 8,wherein said fluid silicone prepolymer composition comprises: a vinylfunctional polydimethylsiloxane, a hydrogen functional siloxane, ahydrosilylation catalyst, and a polymerization inhibitor that isevaporated from said composition during said step of thermally partiallycuring.
 12. A method according to claim 8, further comprising applyingupper and lower release sheets over said silicone composition on theupper and lower surfaces.
 13. A method according to claim 12, whereinsaid upper and lower release sheets are applied intermediate said stepsof thermally partially curing and storing.
 14. A method according toclaim 8 for the production of a wound dressing material comprising: asubstrate layer having an upper surface and a lower surface; a tackysilicone coating composition present on said upper surface and on saidlower surface; and upper and lower release sheets covering saidsubstrate and said tacky silicone coating composition on said upper andlower surfaces respectively, and adhered to said surfaces by said tackysilicone coating composition, wherein said upper surface is less tackythan said lower surface whereby said per release sheet can be removedfrom said upper surface more readily than said lower release sheet canbe removed from said lower surface.
 15. A method according to claim 8further comprising the step of packaging the intermediate material in amicroorganism-impermeable container prior to said step of furthercuring, and wherein said step of further curing also sterilizes saidmaterial.
 16. A wound dressing material according claim 3, wherein thetackiness of said lower surface as measured by a loop tack test is atfrom 20% to 100% greater than the tackiness of said upper surface.
 17. Awound dressing material according to any claim 3, wherein said siliconeon said upper and lower surfaces is substantially chemicallyhomogeneous.